With the progress of semiconductor manufacturing technology, the area or dimensional sizes of components and circuits have been substantially reduced and therefore the operating voltage for driving the components and circuits is also reduced. However, at the meantime when the operating voltage is reduced, the variation range of the threshold voltage which is closely related to the ON/OFF of components is not correspondingly reduced. As a result, the conduction efficiency of some components may not reach to the expectation and consequentially the operating performance of the whole circuit may be affected. Thus, it is quite important to develop a means to control and adjust the threshold voltage of components within a proper range more efficiently.